Over the last few years, a desire for improved data rate and more efficient use of dedicated frequency spectra has led to the proposal and adoption of wideband CDMA as a major standard for next generation mobile telephony. The most popular scheme is that being developed by the 3GPP group, which will be well known to those in the field of mobile telecommunications.
There has been a move over a similar period of time towards delivering data in packet form. One protocol for packet-based transmission is the Internet Protocol (IP) standard. Whilst IP-based transmission of data embodies certain efficiencies, it does have some disadvantages when applied to a mobile telecommunications environment.
For example, the ratio of header information to payload data can often be relatively high. In an internet computing context, such a limitation is of little consequence. However, in a radio communications environment with limited spectrum available, it is desirable to reduce this ratio. One proposal presently being considered is the use of header compression, which would reduce the total number of bits to be transmitted.
Proposed header compression schemes typically still require transmission of an initial or periodic uncompressed header to provide context for the subsequent compressed versions. These uncompressed headers can be relatively large, and, when combined with the payload data and signalling data required for radio transmission, can require the use of relatively low spreading factors. This has the undesirable impact of either reducing available code-space (in a WCDMA system) or requiring high puncture rates.
The most widely adopted specification for the third generation of mobile telecommunications has been proposed, and is further being developed, by the 3rd Generation Partnership Project (3GPP). Code Division Multiple Access (CDMA) has emerged as the most favoured technology for third generation telecommunications, the wideband CDMA (WCDMA) embodiment adopted by 3GPP being known as UTRA (Universal Terrestrial Radio Access). WCDMA, and its application to third generation mobile telephony and the 3GPP standards being developed, is described in some detail in 3GPP Technical Specification 25.212, the contents of which are incorporated herein by cross reference.
In UTPA, transport channels are used to deliver data from higher layers to the physical layers that deal with the mechanics of encoding and transmission over the air. Each transport channel carries data from a different source. This can include, for example, voice data, signalling data used by the system during transmission or IP-based data such as multimedia communications or IP-voice data (along with the header data required for IP communication).
Once the physical layer receives the signal data, it is, amongst other things, spread using a spreading code. This is achieved by multiplying the data with a spreading code, which is pseudo-random in nature and of a particular length, the length of the code being referred to as the spreading factor. For a given bandwidth, a spreading factor 8 would result in data at ⅛ the given chip rate.
Given a restricted bandwidth within which to operate, once packets of data reach a certain size, it becomes necessary to reduce the spreading factor to ensure that bandwidth of the resultant signal falls within the available spectrum.
In the case of IP communications, and IP voice in particular, headers are used for signalling, routing and other ancillary data. The header for each packet can itself require considerably more bits than the payload data within the packet, sometimes by a factor of 2 or more. The present UTRA standard proposed by 3GPP also requires that the header and payload data for a given channel be delivered on the same transport channel and therefore mapped to the same physical channel (or coded composite transport channel).
Spreading of data for an IP voice connection results in some undesirable characteristics. Due to the size of the header, payload and signalling data, it may become necessary to use a relatively low spreading factor to ensure that the encoded signal remains within allocated spectrum. One difficulty with this is that use of a low spreading factor code results in a removal of a relatively large number of orthogonal codes of higher spreading factor from the available code space.